I keep my boat in the garage, but we are having a pretty bad cold spell here in ATL its about 18 degrees outside now. my garage is attached to my house and is probably a little below freezing "maybe" should I be worried? I have never winterized my boat, and thought about running my space heater in there at night just to be safe but I didn't do it last night.

It will take you three hours to figure out how to drain it for the first time and you still might screw it up. Just put a thermometer out there and the heater. Won't take a whole lot to keep it above 32.

it was 20 this morning and my attached garage where the boat is was still in the 40s. It takes a lot to cool a garage attached to the house and then it takes a couple of days below freezing to do damage. I would guess it would need to be in the 20s for a few days. The block is a big chunk of metal basically wrapped in insulation (seats, fiberglass, etc).

With all of that said, I still winterized the block (before I had closed cooling). I can't bet that against a cracked block. You can't count on heaters because you may loose power as well. Realistically I think you will be fine but you never know when you will get caught in the future.

You guys are playing a big guessing game. Here in Northern California the nightime lows often drop down into the teens but daytime highs usually rise up into the 40's so the average is typically over freezing. Based on this I rarely worry about my boat freezing.

I have access to data recorders at work, one or two may even be on permanent loan to me. I have measured my motor compartment temperatures on a few occasions so I am comfortable with my boat outside being without being winterized.

There are several varieties of inexpensive temperature loggers used in the refrigerated trucking industry (not my industry). You might want to spend a few bucks, grab some temperature recorders and run some tests to determine what kind of temperatures your motor compartment is seeing. There are too many variables for any of us to say that your boat is save when average temperatures are down in the teens without looking at the data.